The purpose of this study is to calculate the length and velocity change of gastrocnemius and soleus muscle-tendon complex (MTC) for diagnosis and estimation of the rehabilitation procedure of the patient from non-invasive 3D markers. The previous method measuring the length of MTC has been dependant on the regression equation based on the rotation angle in the sagittal plane. However, in view of the fact that movement analysis is based on the human body having a variety of structure, the measurement using merely rotation angle and regression equation which not based on each subject shank and foot length might not be accurate. In order to overcome these limitations, the length change of MTC is calculated, employing 3D MTC model accompanied with the trajectory data of markers attached anatomical landmarks, each subject measurements and femur condyle radius. Basically, more accurate length change could be acquired through the 3D trajectory data of markers in comparing with 2D data based on the rotation angle. As our study, the difference of the gastrocnemius length change between 3D marker trajectory based method and the method using a 2-D angle was approximately 4% (2cm) at maximum contraction and 1% (0.5cm) at maximum relaxation. Similarly, the difference in terms of the soleus was approximately 0.7% (0.3cm) at maximum contraction and 0.5% (0.2cm) at maximum relaxation.